Motor vehicles with wheel suspension systems which contain an active or semiactive level or height control system for the vehicle body offer various advantages in comparison with vehicles that have a passive wheel suspension system. A constant ground clearance allows an improvement in handling since the compression and rebound movements of the wheels are independent of the loading of the vehicle. It is likewise possible, for example, to raise the vehicle body relative to the underlying surface in urban driving, i.e. at a low driving speed, and thus to increase ground clearance in order, for example, to be able to drive over speed bumps, curbs or underground car park entries with a high ramp angle without problems. At higher driving speeds, the vehicle body can be lowered with the aid of the level control system in order, for example, to reduce the air resistance of the vehicle. In the case of vehicles, in particular passenger vehicles, the movements of the vehicle body are usually damped predominantly by hydraulic means, e.g. by means of a hydraulic oil. As an alternative, however, it is also possible to use air as a working medium to produce damping forces, this having advantages for the environment inter alia in comparison with the use of hydraulic oil. Moreover, the level control systems for vehicles which are commercially available at present are relatively expensive and have a high weight. In addition, they need a large amount of installation space and require a relatively large amount of energy for operation.
Thus, DE 10 2006 055 757 A1, for example, discloses a self-pumping air spring and damper unit operating with compressed air and including automatic level control for vehicle chassis, having a pump which is driven by the variation in the distance between the articulation points of the spring and damper unit on a vehicle body and a chassis caused by the compression and rebound of the vehicle and which produces an increase in the compressed air, as a result of which the air spring and damper unit maintains a defined level in the case of variable loading of the vehicle. The pump is formed essentially of two pump components, which can move relative to one another and which reduce a compression space, of which one pump component is connected to one articulation point and the other pump component is connected to the other articulation point in such a way that the variation in the distance between the articulation points during the compression and rebound of the vehicle is transferred to the distance between the pump components. The spring and damper unit furthermore has two working spaces which can be connected by a throttle valve that allows throughflow and which contain a damping medium that is exchanged between the working spaces, with damping by the throttle valves, during the compression and rebound of the vehicle. One of the working spaces is bounded by a rolling bellows, which rolls on the contour of a rotationally symmetrical rolling cylinder. The compression space of the pump can be connected to one of the working spaces for fluid transmission by a check valve, with the result that, during a compression stroke of the pump or during compression, air is pumped into one working space by the pump. During rebound, air is discharged from the working space via a discharge device that is also provided, which essentially has a discharge cylinder and a discharge piston. The pump and the discharge device are matched to one another in such a way that a constant vehicle height is always obtained, irrespective of the loading of the vehicle, through compression and rebound movements of the vehicle body.
DE 10 2005 060 581 A1 furthermore discloses an air spring and damper unit for vehicles which, as working spaces, has two pressure spaces filled with compressed air and connected to one another by flow channels and has movable walls in the form of rolling or concertina bellows. The compressed air is pumped into the working spaces by a compressor via associated valves and lines, and can likewise be discharged via this system. In one direction of flow, a throttle valve with controllable damping is arranged in a first flow channel and is designed as a pilot-controlled main valve which opens toward the low-pressure side and, on the low-pressure side, can be supplied with a control pressure. The air spring and damper unit disclosed is not capable of performing a self-pumping height adjustment.
DE 43 34 007 A1 discloses a pneumatic spring/damper unit which comprises a cylinder divided by a piston with a piston rod arranged on one side into two pneumatic chambers, the volumes of which change in opposite senses during piston strokes. Moreover, a further pneumatic chamber is provided between components on the piston rod and the adjacent end of the cylinder, the pneumatic chamber being closed by a rolling bellows and normally communicating with one of the two pneumatic chambers via connecting channels in the piston rod. The two pneumatic chambers can be connected to one another via valve-controlled transfer channels provided in the piston. The damper action can be varied by controlling the throttling resistances of the transfer channels. However, DE 43 34 007 A1 does not disclose height adjustment.
DE 44 18 120 A1 describes a pneumatic spring/damper unit very similar to that in DE 43 34 997 A1, in which the valves in the transfer channels can be controlled in accordance with the stroke velocity and/or the stroke travel of the spring strokes in order to be able to vary both the spring stiffness and the damper action.
DE 42 38 790 A1 furthermore discloses an air spring unit having a main chamber, the volume of which can vary during spring strokes, and an additional chamber, which has a substantially constant volume and can be connected to the main chamber or separated therefrom by a switchable valve, thus allowing the progressivity of the spring means to be varied by opening or closing the valve. However, the air spring unit described cannot perform height adjustment or damping.
DE 31 06 122 A1 furthermore describes a height adjustment system for motorcycles in which, with increasing driving speed, compressed air is pumped out of a high-pressure gas source, which is fed by an air pump driven by means of an electric motor, in each case by means of switchable solenoid valves, into a pneumatic shock damper mechanism of a front and rear telescopic suspension in order to raise the chassis of the motor vehicle. As the driving speed decreases, the telescopic suspension systems are vented to atmosphere by means of solenoid vent valves, thus allowing compressed air contained in the shock damper mechanisms to escape. As a result, the chassis of the motor vehicle is lowered.
DE 2 016 540 discloses a springing system for motor vehicles having a mechanical level control system that has a coil spring as a main spring, a gas spring connected in parallel therewith as an additional spring, and a level controller. The level controller is set up in such a way that, when the vehicle is subjected to a load greater than the normal load, the air spring is connected to an excess pressure source and, when the vehicle is subjected to a load lower than the normal load, is connected to a vacuum source. Setting of a vehicle height independent of the loading of the vehicle is thereby achieved.
DE 609723 describes a springing and damping device for vehicles which consists essentially of a metal coil spring and an air spring connected in parallel therewith. The air spring is formed by an air spring cylinder which comprises an outer cylinder, the wall of which is penetrated by bores which allow air to pass inward and outward, and an inner cylinder, which is inserted into the outer cylinder and the wall of which contains machined air passage and throttling bores on both sides of a piston considered to be in the central position and machined air transfer channels on the outer surface. The springing and damping device described is not capable of performing level control.